1. Field of the Invention
This invention relates to underground fluid (liquid and/or gas) collection and production, particularly to coal seam methane gas production and water drainage, and to an improved drainage pattern for gas and liquid collection.
2. Description of the Related Art
Subterranean deposits of coal may contain substantial quantities of entrained fluid, such as methane gas, oil and water. The entrained gas and liquid can be safety hazards for coal mining, especially the methane gas. The removal of entrained gas and liquid can make the coal mining safer and more productive. Although methane gas poses safety concerns in coal mining operations, it is actually one of the cleanest fuels available. Its demand has been increasing steadily. In recent years, methane gas removed from coal deposits has become a useful product in it own right or even a main product. Substantial obstacles, however, have frustrated more extensive development and use of methane gas deposits in coal seams. The foremost problem in producing methane gas from coal seams is that while coal seams may extend over large areas of up to several thousand acres, the coal seams are fairly thin, varying from a few inches to several meters. Thus, while the coal seams are often relatively near the surface, vertical wells drilled into the coal deposits for obtaining methane gas can only drain a fairly small radius around the coal deposits. Further, coal deposits are not amendable to pressure fracturing and other methods often used for increasing methane gas production from rock formations. As a result, once the gas easily drained from a vertical well bore in a coal seam is produced further production is limited.
Additionally, coal seams are often associated with subterranean water, which must be drained from the coal seam at the time the methane is mined. The separation of gas (mostly methane) and liquid (mostly water) is necessary for efficient production or removal of either one.
Horizontal drilling patterns have been tried in order to extend the amount of coal seams exposed to a drill bore for gas extraction. A root type or a pinnate type pattern is generally used. A vertical well located at the center of the pattern, with main bores/branches radiating outwards. Each main bore may in turn have branches to fill the space in between the main bores.
Gases in coal seam may be produced or removed prior to coal mining operation. Vertical well and horizontal bores are drilled. Many of the existing drilling patterns require drilling of several vertical wells in cooperation with horizontal bores in addition of main vertical well. Many of the patterns in the art are not flexible enough to be useful for various field conditions.
It is desirable to have a method and a system to improve the drainage pattern such that the number of vertical wells for a particular field is reduced and the drainage from such field is improved.